The proposed project is a continuation of ongoing work on the physiology of acidurance of dental bacteria. Acidurance is one of the cardinal cariogenic properties of plaque bacteria. The most aciduric plaque bacteria, notably Streptococcus mutans and Lactobacillus organisms, are also the most cariogenic and can reduce the pH value of plaque to slightly below 4.0 so that teeth are subjected to severe acid attack and resulting lesions, which are difficult to remineralize. The acidurance of plaque streptococci was found not to depend mainly in inherent acid resistances of cytoplasmic enzymes but rather on the membrane functions of proton extrusion out of the cell and proton exclusion by membrane lipoidal barriers. Agents which upset proton currents across the membrane greatly reduce acidurance, especially of S. mutans. The proposed project is expanded to consider other cariogenic bacteria - L. casei because of the view that it is active at the leading edge of cariogenic plaque where other bacteria may be inhibited by fluoride or acid, and Actinomyces viscosus because of its involvement in root-surface caries. Work will continue with S. mutans and S. sanguis. A primary consideration will be of changes in acidurance of whole cells due to changes in DeltapH and Deltapsi across the cell membrane, in environmental levels of calcium, magnesium, sodium, potassium, weak acids, including bicarbonate, and in oxidation-reduction potential. This work will be coupled to assessments of proton permeabilities of the cells and functioning of membrane ATPases in isolated membranes, permeabilized cells or vesicles. There will be a focus on the biochemistry and physiology of ATPases because these enzymes appear to be the major engines of acidurance. However, other mechanisms for proton export, including metabolic acid extrusion, will be assessed. The capacities of cariogenic bacteria to adapt to acid enviornments will be explored by use of continuous culture and physiologic characterization. Finally, we shall determine the bases for acid inhibition of glycolysis and growth with focus on acid sensitivities of transport reactions. The ultimate aim of the work is to find means to reduce acidurance through chemical agents or use of implantable, less aciduric organisms which still can compete with other plaque bacteria.